Method for producing differential thickness steel plate

ABSTRACT

A method for producing a differential thickness steel plate having a step portion at least on one side by hot rolling a steel material comprising reversing the hot rolling at the half-way region of the steel material and effecting the rolling with a finishing temperature in a temperature range of 680* to 1050*C.

United States Patent [191 Sakai et al.

[4 1 July 29, 1975 METHOD FOR PRODUCING DIFFERENTIAL THICKNESS STEEL PLATE Inventors: Kazuo Sakai; Ken Iwanaga; Kazuo Maehara; Tsuyoshi Nakajima, all of Kisarazu; Koki Yotsuya; Humio Ookuma, both of Kimitsu; Takeo Baba; Toshitaka Yamanaka, both of Himeji, all of Japan Assignee:

Filed:

Nippon Steel Corporation, Japan Sept. 14, 1972 Appl. NO.: 289,167

Foreign Application Priority Data Sept. 18, 1971 Japan 46-72886 Sept. 18, 1971 Japan 46-72887 Dec. 3, 1971 Japan 46-97161 June 5. 1972 Japan... 47-55868 Sept. 18. 1971 Japan 46-85299 US. Cl. 72/365 Int. Cl BZIb I/OO Field of Search 29/DIG. 32; 72/189, 221,

[56] References Cited UNITED STATES PATENTS 499,677 6/1893 Daclen 72/252 1384,485 7/1921 Putnam 72/197 1.771728 10/1930 Kumpf 72/189 2.161.065 6/1939 Krause 72/214 2,701,976 2/19'55 Krausc 72/252 2.775.152 12/1956 Krausc 72/220 3.611.775 10/1971 Gabcl ct a1. 72/193 Primary Examiner-C. W. Lanham Assistant ExaminerE. M. Combs Attorney, Agent, or Firm-Toren, McGeady and Stanger [57] I ABSTRACT A method for producing a differential thickness steel plate having a step portion at least on one side by hot rolling a steel material comprising reversing the hot rolling at the half-way region of the steel material and effecting the rolling with a finishing temperature in a temperature range of 680 to 1050C.

6 Claims, 18 Drawing Figures PATENTEDJULZQIHYS 3,896,653

FIG.3

PATENTED JUL 2 91975 W, ShLLI 2-! zll %O O O O O O O FIG. 4

FIG.5

O O O O O FIG. 6

O O O O O O O O O FIG.7

PATENTEU 3,896,653

SHEET "I (PLG) MILL ROTATION LOAD CELL METER REVERSIBLE L% I ROLLING MILL o I (TFR) OQOOOOOO 000033099 TABLE NORMAL OR REVERSE .J BUTTON ROTATION ROTATION (RB p5 L SEQUENCE EEQUENCE/ T s CIRCuIT I IRCuIT 2 DEVICE COMPUTER (LD) LENGTH (MFR) MILL NORMAL OR (CPU) INDICATOR REvERsE ROTATION FIG. l2

PLG) (LC) MILL ROTATION LOAD igLL METER REVERSIBLE ("O ROLLING MILL METER OOOOOOOOO QOOOOOOOO (L01) LENGTI I EIDICATORS LENGTH (MRS) INDICATOR I BUTTON IIIlILL TURNING LENGTH MACHINE INDICATOR 2 0 L WCOMPUTER L INDICATOR 3 (CPD) METHOD FOR PRODUCING DIFFERENTIAL THICKNESS STEEL PLATE SUMMARY The present invention relates to a method for manufacturing differential thickness steel plate which is useful for ship building or constructing structures.

Conventionally a flat steel plate having even thickness in width and length directions has been produced for ship building or structure construction as every body knows. In case of structures such as ships, tanks, sluices or chimneys which are to be loaded with static pressure and constructed with the conventional fiat steel plate, they are constructed in such process that flat steel plates of a given dimension are used for the upper part of the structures by welding so as to minimize the weight of the upper part, and other flat steel plates of smaller thickness than that of the foregoing steel plates are used for the lower part of the structures by welding with each other to obtain a required length and obtain higher mechanical strength at the lower part than that at the upper part of the structure.

However, the construction process mentioned above contains not only various problems of repetition of working stages in the method, higher processing costs and complicated treatment, but also an unavoidable disadvantage of poor strength in the quality of material.

Now, construction of bulkheads for partitioning tanks in an oil tanker will be explained hereinafter. Several kinds of operations such as, (l) edge preparation of welding portions, (2) straightening the butting contact edges of steel plates to be welded, (3) butt welding the beveled portions in butting contact with a each other, (4) welding of reinforcement, and the like, have to be carried out in turn, and are very troublesome, in particular, in the operation of items (2) and (4) described above.

In other words, the process for butting the contact edges is required not only to maintain the gap for butting at the beveling portions but also to adjust and secure carefully, for instance, the linear property, surface smoothness, and right angle of edges of thick steel plate to be seamed.

Thick steel plates to be welded together have to be arranged to the fitting position with the aid of a machine tool such as a jack or crane, since the plates to be fitted are rather heavy and, in addition, high technical skill anda fairly long time for the fitting operation is required.

For the welding operation of reinforcement, such as a stiffener, etc. to the welded differential steel plate mentioned in the foregoing item (4), the reinforcement has to be previously worked on the fitting surface thereof to form a common shape to the stepped portion of the steel plate, if the surface of the steel plate to be welded with a reinforcement is formed in a stepped plane.

In order to avoid the aforementioned troubles, in a process previously carried out the steel plates of different plate thickness are welded after adjustment of their height so as to form a level surface on one side face and the level surface portions of welded thickness steel plates are welded with a reinforcement. However, in the above-mentioned process, it is very troublesome, after the welding operation of thick steel plate with a reinforcement. to arrange previously for fitting the steel plate having a smaller plate thickness to float above the work surface in order to avoid forming stepped butting portion of a thick steel plate on the surface to which reinforcement is being welded for the second time.

A reversing operation is required for a thick steel plate of huge dimension, in case a thick steel plate is welded and a reinforcement is given to the flat welding portion of the back surface, and the stepped thick steel plate is turned over.

Increased welding cost is brought and checking for detecting deterioration of the material due to heataffected inclusions is necessitated, as the weld line becomes longer, thus it happens that it is necessary to test several welded portions by supersonic flow detecting and X-ray inspection.

It is necessary to solve the problems, which of necessity require high technical skill, of complicated handling of the material to be treated and deterioration of the material in order to obtain a differential thickness steel plate of a long dimension by welding thick steel plates which are cut off at a given dimension.

A method has been proposed for manufacturing tapered plate which is continuously decreased in plate thickness thereof with even gradient for the purpose of solving the foregoing problems. However the method has been found to be extremely difficult and costwasting at the present stage of industrial production because of the necessity of technical developments, such as automatic mechanical control for thickness of rolled plate.

The present invention is to provide a novel and industrially excellent method, while eliminating the abovementioned drawbacks, for manufacturing differential thickness hot rolled steel plate.

Features of the present invention reside in a method of producing a differential thickness steel plate having a step portion at least on one side, by hot rolling a steel material comprising reversing the hot rolling at a half way point of rolling steel material and effecting the rolling at a finishing temperature in the temperature range of 680 to 1050C.

BRIEF DESCRIPTION OF THE DRAWING The present invention will now be explained with reference to the accompanying drawings in which FIGS. 1-3 are diagrams respectively showing hot rolling of steel plate treated by biting-in the tail end portions for producing of a differential thickness steel plate with steps on both surfaces thereof, according to the present invention.

FIGS. 4-7 are diagrams, respectively, showing hot rolling of steel plate by biting-in the front end portions for producing a differential thickness steel plate with steps on both surface thereof according to the present invention.

FIG. 8 shows hot rolling of steel plate by biting-in the tail end portions as well as flat rolling on the lower surface of rolled surfaces by means of a table roller for producing a differential thickness steel plate with steps on one surface thereof according to the present invention.

FIGS. 9 and 10 show flat rolling of the lower surface of the rolled surfaces by means of a table roller by biting-in the front end portions of steel plate for producing a differential thickness steel plate with steps on one surface thereof according to the present invention.

FIGS. 11a lle, l2 and 13 are schematic sketches respectively show a rolling control device applicable in carrying out the manufacturing process of steel plate according to the present invention; and

FIG. 14 shows an example of a differential thickness steel plate with steps on one surface thereof manufactured according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION In the case of manufacturing a thick steel plate in which the steel plate to be rolled at very high temperature is hot rolled, the present invention is characterised in that after the finishing rolling operation for rolling conventional thickness plate, a differential thickness rolling operation is carried out while the steel plate rolled in the finishing rolling process is kept at a temperature higher than that of the hot rolling operation, and the rolling operation is carried on from the end portion of the steel plate, and subsequently, reversing of the rolling mill taken place at the midway point of the steel plate for rolling it in the reverse direction so as to begin the rolling by biting-in the end portion of the steel plate for producing a differential thickness hot rolled steel plate.

One of the features of the present invention lies in connection with the method of manufacturing differential thickness steel plate (steel plate with a stepped portion) by means of a half-way reversing hot rolling process.

As a method for manufacturing a differential thickness steel plate the following three processes have hitherto been proposed. One of them is a half-way biting-in process, the second one is a half-way fin meshing process and the third one of which is a half-way roll gap process. However, the first two processes are not favorable due to a prolongation in discontinued period of the roll, which causes a fire crack on the roll in contact with hot rolled steel plate during the discontinued period, or decreased efficiency of production through the forming of an empty path, and furthermore lack of protection for bearings of the roll.

In the half-way roll gap variation process forming no empty path, such troubles are caused as unclear indication of the starting point of forming differential thickness and difficulty to obtain precise dimensions.

On the contrary, in the half-way reversing rolling process according to the present invention, rolling reduction for differential thickness is carried out during the period of previous roll rotation of the rolling mill, so that the steel plate is finished high temperature and the rolls are not affected by local heat load for a relatively long period of time.

According to the half-way reversing rolling process of the present invention, there occurs very few problems such as roll cracks or defects in the roll bearings.

As a requirement for producing the differential thickness plate according to the present invention, the present invention is characterized in that the finishing temperature of the steel plate is maintained, at the lowest, between 680 to I050C. Namely, in case of producing the differential thickness steel plate having a step on both sides, the half-way reverse rolling is effected at a finishing temperature between 680 and 850C. In case of producing the differential thickness steel plate having a step only on one side, the plate is subjected to the reverse rolling at a finishing temperature between 850 and 1050C.

The present invention will now be elucidated with reference to FIGS. 1-3 in an embodiment of the invention applied to the manufacture of steel plate having a stepped portion on both surfaces thereof.

Thick steel plate 1 is displaced at the bitingout side 2" of the rolls 2 to be bit in at the tail end portion 1' of the steel plate 1 for rolling the steel plate 1 of a desired length, and treated by the half-way reversing rolling process. Thereby the steel plate is rolled through the hot rolling mill 2 into a form of a stepped steel plate 3.

A roughly rolled steel plate 4 is fed subsequently for the finishing rolling and stands by at the bite-in side 2 of the hot finishing rolling mill 2, and then is immediately put under the finishing rolling operation.

According to the present invention illustrated in the embodiment as shown hereinbefore, the increased efficiency of rolling as well as noticeable effect for decreasing noises during the conveyance operation is obtained.

In the present example, the reason for limiting the finishing temperature of the steel plate between 680 and 850C is that below 850C it is difficult to effect the finish rolling while above 850C the thin portion droops down and thus it is difficult to obtain a step portion on both side of the steel plate. The preferable finishing temperature range is 750 to 850C.

The present invention will now be explained definitely with reference to the drawings in an embodiment of the invention.

An ordinary steel slab having 2,000mm in width, 150mm in thickness and 3,000mm in length is heated in a continuous heating furnace to a temperature of l,250C and is extracted from the furnace to be placed on a roller table. The hot slab thus obtained is treated through a scale breaker to be removed of scale, then rolled into a thick plate product (roughly rolled steel plate 4 to be treated by a finishing rolling roll) having 2,500-mm width, -mm thickness and 4,200-mm length by means of a four-high reversing rough rolling mill, subsequently being subjected to a finishing rolling by means of a four-high reversing finishing rolling mill 2 into a thick plate 1 having 2,500-mm width, l9-mm thickness and l8-m length.

The aforementioned hot thick plate 1 was subjected to the half-way reversing rolling process by means of the four-high reversing mill 2.

The thick plate 1 having 19-mm thickness is placed by the outlet 2" of the finishing rolling mill so as to be treated by the half-way reversing rolling. The finishing rolling mill is set in such a manner that the work roll 2-1 is provided with a gap having 13.3-mm clearance, and the work roll has 30 rpm. rotating rate and 4,000 tons reduction force. thus the product is bit-in, then comparing the pulses from a pulse generator connected with a mill motor of the work roll 2-1 with reference pulse having a standard length for the half-way reversing, and when the number of pulses are equivalent to each other, a control signal for reversing the work roll is emitted. The rotating rate of the work roll is increased at the position of 8 m from the bit-in end without variation in a roll gap reverse rolling. Thus the final finishing temperature of 800C is obtained and the rolling operation is completed in two passes. In this way, it is possible to obtain a product having differential thickness end surfaces in symmetrical configuration on both of the upper and the lower surfaces of the steel plates with the thinner portion 3" having even thickness of l4-mm thickness (said thinner portion having 8,000-mm plate length from the starting end of the step) as well as a steel plate with the thicker portion 3" having l9-mm plate thickness (said thicker portion 3" having 12,000-mm plate length from the starting end of steps forming a are shaped taper part of 500-mm radius 3' nearly at the center of the plate. Subsequently, the product was corrected by a hot levelling machine and was cooled on a cooling floor, and then cut in a predetermined dimension.

Next, examples of the present invention for producing differential thickness plates having a step on both sides will be explained by referring to FIGS. 4 to 7.

In the drawings, 2 is the hot rolling roll, Us a material to be rolled, 3 is a thick portion, 3 is a step portion between the thick portion and a thin portion, 5 is a roller table, 6 is a roll of the roller table.

In FIGS. 4 to 7 the right side of the hot rolling roll 2 is the outlet side 2" and the left side is the inlet side 2.

In FIG. 4 the material 1 is shown to be rolled to a predetermined thickness, and it is passed in the direction of the arrow A between the rolls 2 provided at a certain roll space. Then the material is reversed into the contrary direction of the arrow B in the course of the rolling as shown in FIG. 5 and is taken out of the inlet side as shown in FIG. 6. In this way, step portions 3' are produced on the back and front surfaces of the steel plate I and the thin portion 3" and the thick portion 3 are produced on the outlet side and the inlet side respectively.

Then as shown in FIG. 7, the space between the rolls 2 is opened, and the material is advanced in the direction of the arrow C with the thin portion 3" ahead.

Detailed explanations of the above example are omitted because the explanations made in connection with the preceding examples are applicable and similar results are obtained.

As mentioned above, in the previous examples of the present invention, the material is rolled from its one end and in the course of the rolling, the rotation of the work roll 2-1 of the finishing hot rolling mill is reversed and the bite-in end for the differential thickness rolling is used as the bite-out end so that the starting section of the step portion has a arc-like taper 3' corresponding to the arc of the rolling roll and apart from this taper section the thick portion 3 having substantially no thickness difference continues. Therefore, in the differential thickness plate produced by the halfway reversing rolling method of the present invention, the step portion or the starting point of the thickness difference can be clearly observed by the naked eye and it is possible to obtain final products accurately meeting sizes specified by users.

As an embodiment of the present invention, the halfway reversing rolling is divided in stepwise to delay the starting point of the reversing in the lengthwise direction. In this way differential thickness plates having up to three or four stcp portions can be obtained. The material to be rolled by the present invention may be special steels so far as no welding problem exists.

The differential thickness plate produced by the present invention is mostly 4.8180-mm in thickness in the thin portion, 8200-mm in thickness in the thick portion and the radial length at the arc tapered portion is about 500mm, the thickness difference between the thick portion and the thin portion is 01-20mm, the

thickness ratio of the thick portion to the thick portion is l 0.3 or more, and when the condition of thickness difference 0.1-20mm is satisfied, very excellent results are obtained in mechanical strength and other properties. Particularly, in the differential thickness plates produced by the present invention, as the step portion extends in an arc like taper, not only a very beautiful shape and excelleht functional fitness are obtained, but also there is no concentration of stress in the product, there is no chance of deposition of dust, or rust formation and thus very excellent advantages are obtained in view of rust prevention and strength improvement.

Next, another example of the present invention for rolling a differential thickness plate having a step on one side shall be described hereinafter.

In this example, for the production of thick steel plate by hot rolling high temperature steel plate, the finishing temperature of the differential thickness rolling is maintained'between 850-1050C (not including 850C) so that one of the rolled surfaces is almost flat and the other rolled surface has a step or tapered portion. For high efficiency of rolling, it is within the scope of the present invention to combine the bite-in from the end of the steel plate, the bite-out from the bite-in end by the half-way reversing rolling and the finishing temperature of the differential thickness rolling all together.

The differential thickness steel plate according to the present invention is obtained by maintaining the finishing temperature between 850 and 1050C after the material is rolled by the rolling method shown in FIGS. 1 to 3 or FIGS. 4 to 7 and by transfering the rolled steel plate on the roller table, for example to flatten the plate surface contacting the roller table.

This will be described in details by referring to FIG. 8 and FIGS. 9 and 10.

In FIG. 8, the differential thickness steel plate 3 having step portions on both sides is transferred on the roller table with the thicker portion ahead. In this case the thin portion 3" of the steel plate 3 droop down by its gravity and contacts the roller 5 and the step portion 3' also droops down by its gravity so that the lower rolled surfaces of the thick'portion, the step portion and the thin portion makes alflat line. In this way, a differential thickness steel platelhaving a step portion on one side is obtained. In somecases it is desirable to level thus obtained steel plate by knock-down rolls for example to adjust the shape of the plate.

In FIG. 9, the step portion on the back side of the steel plate 3 having step portions on both sides finished at a temperature between 850 and 1050C is made flat during the transfering the steel plate withthe thin portion ahead on the roller table, contacting a number of rollers on the step portion. In this way, a differential thickness steel plate having a step portion 3' on the upper side but no step portion on the back side as shown in FIG. 10 is obtained.

The differential thickness steel plates produced by the rolling methods shown in FIG. 8 and FIG. 9 are shown in FIG. 14.

The reason for limiting the finishing temperature of the steel plate between 850 and 1050C for the production of a differential thickness steel plate having a step only on one side is that below 850C the thin portion droops down and the flattening of the lower side by the roller table is not satisfactory while above 1050C the material qualities of the steel plate are deteriorated. The preferred finishing temperature is between 900 and 950C.

One preferable example of the differential thickness steel plate having a step on one side according to the present invention is illustrated FIG. 14.

In FIG. 14, the arc tapered portion 11 is positioned at a position between one-fourth and threefourths of the total length of the plate. The right and left portions to the tapered portion I I are respectively a thin portion 13 and a thick portion 12. The thickness rate of the thin portion (13) to the thick portion 12 is l l.2 to 1.5. Abovethis ratio, the rolling operation will be difficult. The most preferred thickness ratio is 3 4, and a pre' ferred thickness difference between the thin portion and the thick portion is 0.1 mm. In practical application, it is desirable if the thickness of the thin portion is 4.8 to 180mm with the above thickness difference and ratio.

The bending radius of the are tapered portion is 400 to 600mm. The continuation from the flat portion of the thick portion to the tapered portion is bordered by a converting point 14. This converting point 14 is very useful for measuring the length of the thin portion.

Next, a controlling method applicable to the rolling of the differential thickness steel plates having a step portion on one or both sides will be described hereinafter.

FIGS. 11 to 13 show an embodiment of controlling in a rolling the differential thickness steel plate having a step portion on both sides by biting-in of the tail end of the steel plate.

The drawing shows that in a steel plate SL of H, in thickness and l in length, the thickness of one side becomes H,-, through H and the length 1 becomes L, I, through the condition of 1 l,, and thus, a differential thickness hot-rolled steel plate SL, having the thickness of H, H,, and the length of l, I is manufactured.

The initial data, necessary for a differential thickness rolling, are memorized in a computer by any process (for example, data transmission, cord paper tape, etc. The dimensions of finished goods, shown by in FIG. llc, namely, the thickness H,, H;,, the length l,, l,, and the width W are given numbers. Therefore, the screw value S and the length 1 can be obtained in the following manner.

In FIG. Ila-e, the known values are H,, H,,, l,, and l The unknown values are H 1 F,, F and S. Here, F, is a rolling reaction force at the time of b and F a rolling reaction force at the time of d.

Among these values there are established the following relative formulas a theoretically and experimentally well-known fact.

H,, s F2/M K F, F (R H, H T,)

(III) In the above, M is a mill constant; R, the radius of the roll; and K, a constant. T, is the temperature of a steel plate, measured by a thermometer, or the temperature, estimated on the basis of the previous pass. T is the temperature, obtained by calculation of the amount of temperature drop from T,.

The number of the above-mentioned equations is 5, while the number of the unknown values is also 5. Therefore, the length l and the screw value S can be calculated in the following manner. By (I) and (II),

Substituting this equation by the equations (III) and (IV).

The unknown value H becomes,known by this equation.

Next, the thickness H is substituted in the equation (III) to get the rolling reaction force F,. F, is substituted in the equation (I) to get the screw value S.

Further, the thickness H is substituted in the equation (V) to get the length 1 FIG. 12 shows an example of the apparatus, by which a differential thickness hot-rolled steel plate is actually manufactured in utilization of these numbers. In the drawing, R is a roll ofa reversible rolling mill; PS, a preset device for performing a screw setting S; SCI and 8C2, sequence circuits, despatching the commands of the table normal or reverse rotation TFR and the mill normal or reverse rotation MFR; CPU, a computer; PS, a button for ordering the differential thickness rolling; LC, a load cell, PLG, a mill rotation meter; TM, a thermometer', and LD, a length indicator. In this apparatus. a steel plate is first rolled by an ordinary rolling process till it gets the thickness H,, and is placed on the rear surface of the mill, as shown by (a) in FIG. 11. The button PS for ordering the differential thickness rolling is pushed by a roll operator or the code for instruction of the differential thickness rolling is put in beforehand. So, the screw value S and the length 1 are calculated by the computer CPU on the basis of the temperature, actually measured by the thermometer, in consideration of the next pass, performing the differential thickness rolling.

The screw is set to S and the mill table is so rotated that the steel plate is bitten by the mill. At the time when the steel plate SL is bitten by the mill, it is started on an ON signal of the load cell LC to count the rotation of the mill through the output of the mill rotation meter PLG. When the rotation reaches the equivalent amount of the length l the mill table is reversely rotated and the steel plate SL is taken out of engagement. Or, the desired point, distant from the centre of the mill by the length of the thick part of the plate, is illuminated by a photo-indicator. At the moment when the end of the plate reaches the illuminated line, the mill is reversely rotated and the steel plate SL is taken out of engagement. As the result, the hot-rolled steel plate of the prescribed differential thickness can be obtained. Further, at this time, the length 1 and the measured value l;, are indicated by the indicator LD, so that the actual result of rolling can be discerned.

FIG. 13 shows an embodiment example. in which the rolling is manually carried out by a roll operator in response to the indication of the computer. In this apparatus, an ordinary rolling is carried out till it becomes in the condition. shown by (a) in FIG. ll. If the roll operator pushes the button PB for ordering the differential thickness rolling. the computer CPU measures the temperature of the steel plate and calculates the length 1 Starting on the ON signal of the load cell, the rotation of the mill is counted. The reversal timing is advised to the roll operator by the indicator LD for instruction of the mill reverse rotation. The mill reverse rotation is detected by the rotation switching signal of the mill turning machine MRS. The measured lengths l l are indicated in the indicator LD to advise the roll operator the actual result of rolling.

As the indicating device LD for instruction of the mill reverse rotation there may be enumerated such as an analog indicator, for which there are used the pulses of the mill rotation number. starting on the starting signal of the computer, or a lamp indicator, provided with several lamps to indicate at suitable intervals for instruction of the timing of reverse rotation. The command from the memory of the computer may be used instead of pushing the button for ordering the differential thickness rolling. The screw value is indicated in the indicator LD,.

According to the above-described method, the screw value S and the length 1 are calculated by the computer to despatch the command of setting and reverse rotation. so that the plate thickness H and the length L, can be determined very exactly. Namely, when the steel plate is pulled out by the reverse rotation of the mill, the screw value is not changed. But, the rolling is con tinued at that time. The plate thickness is reduced from H: to H and the length of the steel plate is stretched from 1 to 1 The differences AH H H and AI l 1 are cited as an instance. as follows: If, H

l5.2mm, H, l4.l5mm, l ll,l60mm and l l0 300mm, it results that AH 1.05mm and A] 860mm.

It can be found from this result that if the command of screw setting and reverse rotation is despatched without regard to the differences AH and Al. there may be an error. corresponding to the differences. However, such an error is not made according to the above described method. Further, the timing of reverse rotation can be directly commanded to the driving gear from the computer. when the length of the steel plate reaches l- Or the instruction may be given to the instruction device and the worker can promptly despatch the command of reverse rotation. Therefore, a very exact determination can be performed as compared with a method. comprising that a mark is given on the line. so that the timing of reverse rotation can be checked.

A complete example of the present invention will now be described. This example relates to production of a differential thickness steel plate 14mm thick in the thin portion. and 17mm thick in the thick portion and 9.800-mm length in the thin portion and 6500-mm length in the thick portion.

The molten steel having the following composition was prepared in a converter and cast into slabs of 210- mm thickness, 1,900-mm width and 2,700-mm length by a continuous casting.

Steel Composition Si 0.13 0.22 1.00 0.02l 0.010

The slabs were heated to [260C in a continuous heating furnace. The slabs taken out of the heating furnace were transferred on a roller table and removed of surface scales, and rolled by a reversible four-high rough rolling mill to a size of 7 l-mm thickness and 3,260-mm width. These rough rolled materials were further rolled in a finishing rolling mill. In the finish rolling mill, first the material was rolled into a plate thickness in the thick portion, and then the screw value was set at 13.0 m/m to bite in the material with the thin portion ahead at a mill speed of 0.9 m/sec. and at a predetermined plate length, the mill was reversed with the screw fixed. In this case the accuracy of the plate thickness was determined by a standard deviation (between the actual thickness and the aimed thickness). The results showed very good accuracy as compared with that of the ordinary rolling.

In this example. the rolling was done by one back and forth rolling, but in case ofa larger thickness difference where larger reduction force is required, the rolling may be done in two or three back and forth rollings.

The differential thickness steel plate was thus attained at a spped of 150 m/sec. transferred on a m length roller table having rollers of 400mm diameter, 4,700mm length and 900mm roller pitch. During the transferring on the roller table the lower side of the differential thickness steel plate was flattened. Then the plate was subjected to a hot leveller to adjust the shape and cooled on a cooling floor.

What is claimed is:

1. A method for producing a differential thickness steel plate having a step portion at least on one side by hot rolling a steel material through a rolling mill comprising adjusting the rollers of the rolling mill to a preset roll gap, conveying the steel material to the rolling mill, rolling the steel material commencing with an end thereof in the rolling mill, reversing the direction of rolling of the steel material after a specific portion thereof has been rolled, and finishing said rolling with a temperature range of 680 to lO50C whereby a differential thickness steel plate is formed.

2. A method according to claim 1, wherein said temperature range is between 680 and 850C, and said step of reversing the direction of rolling comprises forming a step portion on both sides of the steel material.

3. A method according to claim I, wherein said temperature range is between 850 and lO50C, and said step of finishing said rolling comprises forming a step portion on only one side of the steel material and a flat, continuous portion on the other side thereof.

4. A method according to claim 3, wherein said rolling of the steel material commences with the tail end thereof. and said step of reversing the direction comprises reversing the direction at the half-way region of the steel material.

prises forming a differential steel plate having a step portion in both sides thereof, and said step of finishing comprises transferring the steel plate to a roller table so to form a differential steel plate with a step portion on only one side thereof. 

1. A method for producing a differential thickness steel plate having a step portion at least on one side by hot rolling a steel material through a rolling mill comprising adjusting the rollers of the rolling mill to a preset roll gap, conveying the steel material to the rolling mill, rolling the steel material commencing with an end thereof in the rolling mill, reversing the direction of rolling of the steel material after a specific portion thereof has been rolled, and finishing said rolling with a temperature range of 680* to 1050*C whereby a differential thickness steel plate is formed.
 2. A method according to claim 1, wherein said temperature range is between 680* and 850*C, and said step of reversing the direction of rolling comprises forming a step portion on both sides of the steel material.
 3. A method according to claim 1, wherein said temperature range is between 850* and 1050*C, and said step of finishing said rolling comprises forming a step portion on only one side of the steel material and a flat, continuous portion on the other side thereof.
 4. A method according to claim 3, wherein said rolling of the steel material commences with the tail end thereof, and said step of reversing the direcTion comprises reversing the direction at the half-way region of the steel material.
 5. A method according to claim 3, wherein said step of forming a flat portion comprises contacting the other side of the steel material with a roller table whereby the steel material is caused to level out and form a flat portion due to the force of gravity.
 6. A method according to claim 1, wherein said step of reversing the direction of the steel material comprises forming a differential steel plate having a step portion in both sides thereof, and said step of finishing comprises transferring the steel plate to a roller table so as to form a differential steel plate with a step portion on only one side thereof. 